The present invention relates to fiber amplifiers, and more particularly, to a multimode fiber amplifier which discriminates against higher-order modes (e.g., discriminates against all but the fundamental mode).
Single-mode (SM), rare-earth-doped fiber lasers and amplifiers are finding widespread use in applications requiring compact, rugged optical sources with diffraction-limited beam quality. The advent of double-clad fibers has allowed these sources to be scaled to average powers of  greater than 100 W. (See V. Dominic, S. MacCormack, R. Waarts, S. Sanders, S. Bicknese, R. Dohle, E. Wolak, P. S. Yeh, and E. Zucker, in Conference on Lasers and Electro-Optics (Optical Society of America, Washington D.C., 1999), paper CPD11). For applications requiring high-energy pulses, such as nonlinear frequency conversion (see J. P. Koplow, D. A. V. Kliner, and L. Goldberg, IEEE Photon. Technol. Lett. 10, 75 (1998)), pumping of optical parametric oscillators/amplifiers (see P. E. Britton, H. L. Offerhaus, D. J. Richardson, P. G. R. Smith, G. W. Ross, and D. C. Hanna, Opt. Lett. 24, 975 (1999)), Iidar, and materials processing, the use of fiber-based systems has been limited by the relatively low pulse energies available compared to bulk lasers. Similarly, for continuous-wave (cw) applications requiring narrow linewidth, fiber sources are restricted to relatively low power.
These limitations arise from two factors: low energy storage and the onset of nonlinear processes in the fiber. The maximum pulse energy that can be attained with a fiber amplifier (i.e., the amount of energy that can be stored in the gain medium) is limited by amplified spontaneous emission (ASE). The peak- and average-power-handling capability of fiber sources is determined by the onset of nonlinear processes that occur inside the fiber. Both of these limitations are briefly reviewed here.
Energy Storage: The primary loss processes for energy deposited into the gain medium are spontaneous emission (fluorescence) and amplified spontaneous emission. The vast majority of spontaneous emission, which is emitted equally in all directions, escapes out the side of the fiber. However, a small fraction of this light is captured in the core and amplified as it propagates down the length of the fiber (ASE). When the population inversion in the gain medium is relatively low (i.e., at low pump power), spontaneous emission is the dominant loss process. In this low power limit the stored energy increases linearly with pump power. At sufficiently high pump power, the gain in the fiber becomes large enough that the power lost to ASE is comparable to the power lost to spontaneous emission. As ASE starts to take over as the dominant loss process, the stored energy as a function of pump power begins to level off. Eventually, a high power limit is reached in which the stored energy increases logarithmically (rather than linearly) with pump power. For this reason the maximum amount of energy that can be stored in the gain medium is determined by ASE.
Nonlinear Processes: Nonlinear processes that occur at high optical intensities impose an upper limit on the amount of power that can be transmitted through a given length of fiber. The most important of these nonlinear processes are stimulated Brillioun scattering (SBS), stimulated Raman scattering (SRS), and self-phase modulation (SPM). (See Nonlinear Fiber Optics, G. P. Agrawal, Academic Press, San Diego, Calif, 1995.) These processes are characterized by a threshold power, above which a significant portion of the energy in a high-power pulse is converted to different (unwanted) wavelengths. The relative importance of these processes depends on pulse duration and spectral bandwidth.
Two approaches to overcoming these limitations have been reported. Taverner et al. developed large-mode-area, Er-doped SM fibers with numerical aperatures (NA) of 0.066 to 0.08 and core diameters of 14 to 17 xcexcm. (See D. Taverner, D. J. Richardson, L. Dong, J. E. Caplen, K. Williams, and R. V. Penty, Opt. Lett. 22, 378 (1997); G. P. Lees, D. Taverner, D. J. Richardson, L. Dong, and T. P. Newson, Electron. Lett. 33, 393 (1997)) Decreasing the NA (relative to standard telecommunication values of xcx9c0.15) allows the core size to be increased while maintaining SM operation. The resultant increased mode-field area raises the threshold for nonlinear processes. In addition, the lower NA reduces the fraction of spontaneous emission captured by the fiber, thereby increasing energy storage. (See J. Nilsson, R. Paschotta, J. E. Caplen, and D. C. Hanna, Opt. Lett. 22, 1092 (1997)) Several groups have used multimode fiber amplifiers and have obtained varying levels of suppression of high-order modes by adjusting the fiber index and dopant distributions (See H. L. Offerhaus, N. G. Broderick, D. J. Richardson, R. Sammut, J. Caplen, and L. Dong, Opt. Lett. 23, 1683 (1998); J. M. Sousa and O. G. Okhotnikov, Appl. Phys. Lett. 74, 1528 (1999)), cavity configurations (See U. Griebner, R. Koch, H. Schonnagel, and R. Grunwald, Opt. Lett. 21, 266 (1996); U. Griebner and H. Schonnagel, Opt. Lett. 24, 750 (1999)), and/or launch conditions of the seed beam. (See O. G. Okhotnikov and J. M. Sousa, Electron. Lett. 35, 1011 (1999); C. C. Renaud, R. J. Selvas-Aguilar, J. Nilsson, P. W. Turner, and A. B. Grudinin, IEEE Photon. Technol. Lett. 11, 976 (1999); M. E. Fermann, Opt. Lett. 23, 52 (1998); M. Hofer, M. E. Fermann, A. Galvanauskas, D. Harter, and R. S. Windeler, IEEE Photon. Technol. Lett. 11, 650 (1999); I. Zawischa, K. Plamann, C. Fallnich, H. Welling, H. Zellmer, and A. Txc3xcnnermann, Opt. Lett. 24, 469 (1999).)
An object of the present invention is to provide a multimode fiber amplifier having maximum pulse energy, peak power, and average power capabilities greater than those of conventional single-mode fiber amplifiers while maintaining beam quality comparable to such single-mode fiber amplifiers.
It is a further object of the present invention to provide a fiber amplifier comprising a multimode fiber for supporting the fundamental mode (LP01) while suppressing higher-order modes so as to produce an output beam having a beam quality that is diffraction-limited (or near diffraction-limited).
It is a further object of the invention to provide a multimode fiber amplifier that discriminates between the fundamental mode and higher-order modes, i.e., attenuates the latter to a significantly greater extent than the former.
It is a further object of the invention to provide a multimode fiber amplifier which discriminates comparably between the + and xe2x88x92 helical polarities of the undesired higher-order modes.
These and other objects are achieved by the provision, in accordance with the present invention, of a multimode fiber amplifier configured to have a bend loss providing greater suppression of higher-order fiber modes than of a preselected fiber mode of lower-order than the higher-order fiber modes.
Preferably, the fiber amplifier comprises a multimode fiber that is coiled to achieve the aforementioned mode-filtering effect.
In accordance with a further aspect of the invention, there is provided the multimode fiber amplifier system providing discrimination between a fundamental mode and undesired higher-order modes, the amplifier system comprising: a light source for producing a light beam, and a multimode fiber amplifier for receiving said light beam and comprising a multimode optical fiber, capable of supporting a fundamental mode and higher-order modes, having a radius of curvature such that the higher-order modes experience substantially increased bend losses as compared with the fundamental mode.
Preferably, the fiber comprises a coiled fiber. The coiled fiber preferably has a constant radius of curvature but, in useful embodiments, the coiled fiber can have a non-constant radius of curvature.
In one preferred implementation, the light source comprises a continuous wave light source while, in another, the light source comprises a pulsed light source.
The multimode optical fiber preferably comprises a double-cladding structure. Advantageously, the multimode optical fiber comprises a core having a diameter of between 3 xcexcm and 100 xcexcm (although a core diameter in excess of 100 xcexcm is also possible).
In an important implementation, the multimode optical fiber is coiled onto two mandrels, comparable in diameter, whose longitudinal axes are mutually perpendicular, where the length of fiber wound onto each mandrel is approximately equal.
The multimode fiber amplifier preferably has an M2 value less than 1.2, where M2=1 denotes diffraction-limited beam quality. In some applications, however, operation at an M2 value greater than 1.2 may prove advantageous.
In accordance with a further aspect of the invention, a multimode fiber amplifier is provided which comprises: a cylindrical support member, and a multimode optical fiber having a core diameter such that the fiber is capable of supporting a fundamental mode and a plurality of higher-order modes, the fiber being wound onto the support member having a radius such that said higher-order modes experience substantially increased bend losses as compared with the fundamental mode.
As above, the coil of fiber preferably has a constant radius of curvature, although, in useful implementations the coil of fiber can have a non-constant radius of curvature.
Also as above, the multimode optical fiber beneficially comprises a double-cladding structure, and advantageously comprises a core having a diameter of between 3 xcexcm and 100 xcexcm (although a core diameter in excess of 100 xcexcm is also possible).
In a beneficial implementation, the amplifier comprises a second cylindrical support member having a diameter equal to that of the first-mentioned cylindrical support member, the second support member having a longitudinal axis extending perpendicular to that of the first support member.
Further objects, features and advantages of the present invention will be set forth in, or apparent from, the detailed description of preferred embodiments thereof which follows.